1. Field of the Invention
The present invention relates generally to a terahertz wave generator and a method of generating high-power terahertz waves using the terahertz wave generator. More particularly, the present invention relates to a terahertz wave generator implemented using a hollow spherical body and a method of generating high-power terahertz waves using a plurality of air plasmas in the terahertz wave generator.
2. Description of the Related Art
Terahertz waves denote signals which fall within a frequency range from 0.1 to 10 THz, which equal to a wavelength band from 0.03 to 3 mm. Currently, terahertz waves are widely used in biology, chemistry, national defense, environmental detection, security, and communication, etc. As the generation of coherent terahertz waves (THz waves) becomes possible due to the use of stabilized femtosecond (lfs=10−15 seconds) optical pulses and the recent excellent results of engineering such as material engineering, new research fields different from the flow of millimeter wave or sub-millimeter wave engineering, originating from previous microwave engineering, or typical far-infrared spectroscopy, have been developed.
Terahertz waves can be generated in such ways that are to emit wideband pulse-shaped terahertz (THz) light from a material excited using an ultra-short pulse laser, to use the acceleration of electrons in a photoconductive antenna, to use a nonlinear effect in electro-optic crystals, or to use plasma oscillation.
An ultra-fast pulse laser light can be radiated onto a GaAs or InP semiconductor which is a photoconductor (so that photonic energy is greater than the band gap of a material), and thus electron-hole pairs are generated. When a bias electric field of ˜10 V/cm is applied to such a semiconductor, free electrons and holes are accelerated, and thus photoelectric current is produced. At this time, the accelerated electrons produce THz light. A THz pulse generation device is configured such that a divided antenna is manufactured on a semiconductor substrate to form switches, and such that, when a dc bias is applied to both ends of the antenna and ultra-fast laser pulses (<100 fs) are condensed onto an antenna gap, electrons cross the gap at high speed, and thus the current of the antenna enables THz pulses to be generated. A THz pulse light source using a photoconductor has low output power, but generates stabilized and coherent beams. Accordingly, such a THz pulse light source is used in high-resolution Time Domain Spectroscopy (TDS), and exhibits excellent Signal/Noise Ratio (SNR) in THz imaging technology.
The generation of THz light using the nonlinear effect of an electro-optic crystal is intended to generate THz pulses using the nonlinear effect of a crystal such as GaAs or ZnTe produced when an ultra-fast pulse laser radiates light onto such a crystal. That is, the nonlinear effect in which an incident beam having a frequency of win is divided into two beams respectively having low frequencies of ωout1 and ωout2 is exhibited (ωin=ωout1+ωout2). Frequencies ωout1 and ωout2 undergo into optical rectification process to generate THz light. This method has low efficiency, but is advantageous in that it has a wide bandwidth.
Efficient THz wave generation methods are divided into methods using optical rectification based on an x(2) process, and methods using four-wave mixing based on an x(3) process. THz wave generation methods based on an x(2) process may include methods using optical media such as Zinc telluride (ZnTe), Cadmium telluride (CdTe), and c-cut Diethylaminosulfur trifluoride (DAST), each having strong x(2) characteristics. In addition, methods using gas or liquid are also widely known. When femtosecond optical pulses are propagated into an electro-photo crystal having a high x(2) value, THz pulse waves of about 1 cycle are generated while forming a Cerenkov circle, owing to optical rectification. That is, when femtosecond laser pulses are radiated onto the surface of a semiconductor on which a surface electric field is formed, carriers (electrons and holes) excited by the laser are accelerated because of the electric field on the surface of the semiconductor, and thus a current (called ‘surge current’) flows and a THz pulse wave is generated. InP or GaAs is a semiconductor having a large surface electric field. Such a semiconductor simultaneously radiates THz pulse waves generated by the optical rectification of an incident optical pulse obtained based on a secondary nonlinear optical effect occurring near the surface, that is, x(2) processing.
Further, a representative method based on an x(3) process may be a method using air plasma. The generation of a terahertz wave using air plasma was first posted in 1994 in a publication (H. Hamster, A. Sullivan, S. Gordon, R. Falcone, Phys. Rev. E 49, 671 (1994)). However, it is well known that the light intensity of a terahertz wave is very low, and that the conversion ratio, at which a focused laser beam is converted into a terahertz wave, is very low. In order to solve these problems, methods of increasing light intensity by generating two or more air plasmas were proposed in a plurality of publications (X. Xie, J. Dai, X.-C. Zhang, Phys. Rev. Lett. 96, 075005 (2006) and M.-K. Chen, J H. Kim, C.-E. Yang, S. S. Yin, R. Hui, P. Ruffin, Appl. Phys. Lett. 93, 231102 (2008)). However, the experimental schemes proposed in the above publications are disadvantageous because whenever one air plasma is generated in the case where a plurality of air plasmas is generated, a pair of parabolic mirrors is added, and thus an increase in cost attributable to the addition of parabolic mirrors is very high. Further, when one air plasma is additionally generated, the additionally required space is relatively large.
Therefore, there is required an apparatus and method which uses air plasma based on an x(3) process, but can generate high-power terahertz waves by generating a plurality of air plasmas in a narrow space without requiring additional parabolic mirrors.